1. Field of the Invention
The present invention relates to a combined electricity generation system which combines two methods for generating electricity in which a steam turbine is driven by steam generated from a pressurized fluidized bed boiler and a gas turbine is driven using the exhaust gas from the boiler.
2. Description of the Related Art
Pressurized fluidized bed combined electricity generation systems are electricity generation systems which combine two methods such that electricity is generated by driving a steam turbine with steam generated from a fluidized bed boiler housed within a pressurized container, and electricity is generated by driving a gas turbine by introducing exhaust gas from the pressurized fluidized bed boiler into the gas turbine. For the combustion which occurs in the pressurized fluidized bed boiler, air for combustion is introduced into the boiler in a pressurized state from a compressor, crushed coal is added to the fluidized bed in which limestone forms the fluid medium, and combusted in a fluid state. A steam pipe is arranged within this fluidized bed, steam is generated due to the heat of combustion in the fluidized bed, the steam turbine is driven and electricity is generated. In addition, with this type of combustion, since limestone is used, it is possible to conduct desulferization within the furnace at the same time.
Further explaining the principle of the fluidized bed boiler mentioned above, an air dispersion plate is provided in the bottom of a container and solid particles are charged into the part above the air dispersion plate. Air is blown uniformly from the bottom of the air dispersion plate, and when the amount of air is increased, solid particles move vigorously and randomly within a layer of a certain height above the air dispersion plate. The solid particle layer which is floated and fluidized by a fluid in this way is referred to as a fluidized bed and the combustion in a fluid state of liquid fuel or solid fuel of a suitable size added to this fluidized bed is fluidized bed combustion.
FIG. 9 is a schematic diagram showing an example of the pressurized fluidized bed combined electricity generation system explained above.
In FIG. 9, reference 1 indicates the entire pressurized fluidized bed boiler 1 comprises a pressurized container 10 and a boiler 11 therewithin. A coal/limestone supply device 12 supplies the limestone which forms the fluid medium and the coal starting materials to the boiler 11. A cyclone 13 removes particles which are noncombustible, and the like, from the exhaust gas from the boiler 11. A dust collecting device 14 comprises a ceramic filter, and ash and the like are filtered and removed by this ceramic filter. A gas turbine 15 is directly coupled to the compressor 16, is driven by high temperature exhaust gas from the dust collecting device 14. Also, there is a denitrification device 17, a high pressure exhaust heat recovery/supply-water heater 18 and a low pressure exhaust heat recover/supply-water heater 19. The high pressure exhaust heat recovery/supply-water heater 18 and the low pressure exhaust heat recover/supply-water heater 19 recover exhaust heat from the exhaust gas and preheat the water being supplied to the boiler 11 using this exhaust heat. A chimney 20 discharges the exhaust gas to the atmosphere.
In addition, in FIG. 9, there is a steam turbine 21 and a condenser 22 to which cold water is sent by a pump 51 and in which steam from the steam turbine 21 is cooled and condensed. A low pressure supply-water heater 23 heats the condensed water and regulates the temperature of the supply water. A deaerator 24 removes air bubbles from the supply water. A drum 25 supplies water to each of the pipes of boiler 11, that is, to main steam pipe system 30, reheating gas pipe system 31 and boiler circulation pipe system 32.
In the pressurized fluidized bed combined electricity generation system of the above-described structure, the limestone which is the fluidized bed medium and the coal starting material from the coal/limestone supply device 12 is sent to the boiler 11 by a supply system 41, while air from the compressor 16 is blown into the boiler 11 by an air system pipe 42, a fluidized bed is formed by this limestone and the coal is burned, thereby, fluidized bed combustion is carried out.
At the same time, supply water that has been heated in advance is supplied from the drum 25 to the main steam pipe system 30. The main steam pipe system 30 is heated by the boiler 11, steam is generated and the high pressure turbine of steam turbine 21 is driven. The steam discharged therefrom is returned to the boiler 11 again, it is reheated, run back into the steam turbine 21 a second time by the reheating steam pipe system 31, drives the low pressure turbine, and flows to the condenser 22. In addition, the supply water in the drum 25 circulates between the boiler 11 and the drum 25 by the boiler circulation pipe system 32 such that it is heated.
Next, large particles are removed by the cyclone 13, ash and the like are removed by the dust collecting device 14, then the combustion exhaust gas from the boiler 11 is supplied to the gas turbine 15, the turbine 15 is driven and electricity is generated. The exhaust gas which drives the gas turbine 15 is denitrified by the denitrification device 17. Then the remaining heat in the exhaust gas is used to heat the supply water being supplied to the boiler 11 by each of the high pressure exhaust heat recovery/supply-water heater 18 and the low pressure exhaust heat recover/supply-water heater 19. Thereafter, the exhaust gas is a discharged to the atmosphere from chimney 20.
The exhaust steam which drives the steam turbine 21 is condensed and liquefied by the condenser 22, to which cold water is sent by the pump 51 this condensed water is sent to the low pressure supply-water heater 23 by a pump 52 where it is heated and its temperature adjusted. Then it is preheated by exhaust gas in the low pressure exhaust heat recover/supply-water heater 19 and then is sent to a deaerator 24 and bubbles are removed. The supply water from the deaerator 24 is sent to the high pressure exhaust heat recovery/supply-water heater 18 by a pump 53 where it is preheated again and then sent to drum 25 by a pipe 33.
In the above-described pressurized fluidized bed combined electricity generation system, there is a combined cycle system which generates electricity by driving the steam turbine 21 and by the gas turbine 15 using the exhaust gas from the boiler 11, and it obtains high electricity generation efficiency. In addition, since the boiler 11 is housed inside the pressurized container 10, it can be made to be compact. In addition, since limestone is used as the fluid medium, desulferization can be carried out within the boiler 11, an exhaust gas desulferizer is not necessary, and it is possible to make the plant equipment area smaller compared with the past.
In the above-described pressurized fluidized bed combined electricity generation system, there is a combined cycle method in which a turbine is driven by steam generated by the pressurized fluidized bed boiler 1, the gas turbine 15 is driven by exhaust gas from the pressurized fluidized bed boiler 1, and it is possible to obtain high electricity generation efficiency. It is also possible to make the apparatus compact. In this type of system, the output of the gas turbine 15 is influenced by the characteristics of the fuel used (coal), the combustion method of the pressurized fluidized bed boiler 1 (combustion temperature, layer height), and the like, however, it depends greatly on the temperature of the intake air of the compressor 16 of the gas turbine 15 and the air-fuel ratio for combustion in the pressurized fluidized bed boiler 1.
The temperature of the intake air of the compressor 16 can be varied intentionally by operation using intake of air from indoors/outdoors, but it is basically determined by natural conditions. The air-fuel ratio in the combustion in the pressurized fluidized bed boiler 1 has a certain degree of freedom due to changes in the operating conditions of the pressurized fluidized bed boiler 1, but it is impossible to exceed the upper limit of the compressor 16. Under these types of conditions, it is possible, to some extent, to take measures to increase the output of the gas turbine 15 but, even in situations where the output of the gas turbine 15 is limited for some reason, for example, the temperature of the air drawn into the compressor 16 is higher than anticipated, there are cases (gas turbine certification output tests and the like) where it is necessary to increase the output of the gas turbine 15 beyond what is practically possible using present technology. In these types of situations, at present, it is not possible to cope with increasing output of the gas turbine 15.
In addition, in coal gasification combined electricity generation systems, in the same way, when there is, for some reason, a limitation to gas turbine output, there are cases (gas turbine certification output tests and the like) where it is necessary to increase the turbine output beyond what is possible using present technology, in these situations as well, increases in output beyond what can be responded to using present technology are impossible.